In a multilayered printed wiring board in recent years, a multilayered printed wiring board embedded a capacitor circuit including a capacitor circuit in the inner layer has been employed as a power supply circuit/ground circuit to achieve a uniform power distribution and reduced noise generation. Also, high speed signal transmission in a device, stable power supply to the device for operation, power saving and reduction in noise generation are considered. In particular, such a multilayered printed wiring board embedded a capacitor circuit is one of a solution for mounting of LSI chips having increased wiring density and enlarged size used in a communication apparatus for infrastructure, a network server, a supercomputer and the like.
As a technique related to the multilayered printed wiring board embedded a capacitor circuit, Patent Document 1 discloses a multilayered printed wiring board embedded a capacitor circuit including a capacitor circuit (embedded capacitor circuit) having a layer structure “conductive foil/dielectric sheet/conductive foil” in the inner layer of a multilayered printed wiring board (Patent Document 1 refers to as “capacitive printed wiring substrate”).
If the multilayered printed wiring board embedded a capacitor circuit manufactured by the method disclosed in Patent Document 1 is sued in a communication apparatus for infrastructure, a network server, a supercomputer and the like described above, the voltage of the power supply circuit can be made lower but a large electric current is required. To pass a large current without heat generation in the power supply circuit, increase of thickness of the conductor is required to lower the electric resistance of the conductor. So, in the power supply and ground circuit design, thick copper layer is used to lower electric resistance with a widened cross sectional area of the wiring.
In forming of the capacitor circuit of the multilayered printed wiring board embedded a capacitor circuit disclosed in Patent Document 1 described above, a double-sided copper clad laminate having a layer structure of “copper layer/dielectric layer of the capacitor/copper layer” is preferable to be used. Further, the double-sided copper clad laminate disclosed in Patent Document 2 is preferable to be used to suitably form the embedded capacitor circuit in the multilayered printed wiring board embedded a capacitor circuit disclosed in Patent Document 1.
Patent Document 2 discloses an object, “to provide a double-sided copper clad laminate for forming of the capacitor layer provided with an insulating layer excellent in break down voltage properties without causing a short circuit with voltage application between copper foil and not broken by showering pressure in etching for wiring forming”, and “the double-sided copper clad laminate for forming of the capacitor layer provided with a conductive copper foil layer on the outer layer on both sides and a dielectric resin layer sandwiched between the copper foil layers, wherein the resin layer has a there layer structure composed of thermosetting resin layer/heat-resistant film layer/thermosetting resin layer and a total thickness of 25 μm or less. The thermosetting resin layer is composed of epoxy-based resin material, the heat-resistant film layer as-received has a Young's modulus of 300 kg/mm2 or more, a tensile strength of 20 kg/mm2 or more, elongation of 5% or more, a softening temperature is higher than the laminating temperature of the thermosetting resin constituting the thermosetting resin layers on both sides and the epoxy resin material has a relative dielectric constant of 2.5 or more”.
In general, the capacitance of the capacitor circuit is higher as the dielectric constant of a dielectric layer increases and as the thickness of a dielectric layer decreases. So the thickness of a dielectric layer should be thin as much as possible. However, excessively thin dielectric layer has drawback that the break down voltage between the top electrode and the bottom electrode in the capacitor circuit becomes low and causes a short circuit. In the double-sided copper clad laminate disclosed in Patent Document 2, a strong and heat resistant film is arranged in the dielectric layer. The film in the dielectric layers can surely prevent contact between the top electrode and the bottom electrode of the finished capacitor circuit provided with a thin dielectric layer and assure excellent dielectric strength. So, the double-sided copper clad laminate disclosed in Patent Document 2 is preferable to be used in the invention disclosed in Patent Document 1.